Telephone answering machines are devices that perform call handling on an automated basis. Their limitations, however are well-known. For example, standard telephone answering machines require special circuitry for detecting the off-hook condition of the associated station and cannot provide an answering service when the line with which it is associated is busy. Other call coverage arrangements are known by which a telecommunication switching system such as a central office switch or a PBX switching system on a customer premises forwards incoming telephone calls to another number. The calls may be forwarded to an alternate destination, such as a telephone answer desk or voice mail system, when the principal telephone does not answer, is busy, or has activated a feature forwarding all calls. In one prior art system, a personal computer is interposed between a telephone station and a business communication system on a customer's premises. In that arrangement, the computer intercepts all call signaling from the communication system and creates a record of the calls.
A problem with the prior art arrangements where functions such as call forwarding are performed by the switching system is that they tend to be expensive for smaller companies which may not have a private system on their premises. A problem with prior art arrangements using a computer interposed between a station and the switch on a per-line basis is that the capabilities of the computer are limited to functions that can be performed by monitoring the signal activity on a single line. Such a computer has limitations similar to a standard telephone answering machine in that it cannot provide call handling services while the line with which it is associated is busy.
Voice message recording systems are commonly used in connection with telecommunication switching systems such as central offices or customer premises private branch exchanges (PBXs). Besides receiving a voice message from the telecommunication system, the more sophisticated voice message recoding systems receive information from the telecommunication system, including information identifying the called party or called party directory number. This information is used in the voice messaging system to provide a customized announcement to the calling party and to record the message with indices which allow for easy retrieval. The transmission of called party information may also be used by business offices or service bureaus in which telephones are answered by human operators and messages are recorded by the operator's entry of information provided by the caller in a data base which may be retrieved at a later time by the called party.
Some systems have a large number of users; several thousand users are typically served from a central office by means of a specially designed data link of the switching system over which the switching system transmits the appropriate information to the message recording system. The link may also be used by the recording system to transmit to the switching system information used by the switching system to activate a message waiting light on the equipment of the called subscriber.
A separate data link is not needed when the message recording system is provided with a dedicated line for each client. In that case, the switching system simply transfers the call to the dedicated line in case the subscriber is busy or unavailable and the recording system or service bureau does not need separate information since the identity of the line identifies the client to the call answering and recording system. Such an arrangement is economical only for systems handling a small number of clients. In other systems, typically PBX's, several clients share a single line to the message system and the switching system is modified to outpulse the called number on that line prior to making the voice connection for answering and recording message information. In other less sophisticated message recording systems, no information with respect to the called party is transmitted from the switching system and the calling party, by announcement, is asked to dial (using a dual tone multifrequency (DTMF) station) the called party number and the messages are stored by number. Alternatively, the calling party may be asked to speak the name of the called party and a message is retrieved by listening to the names at the beginning of each message.
A problem of the prior art is that in large central message recording systems, which are much more economical than an aggregate of smaller systems, an especially designed data link adds a substantial expense to the system, particularly at the low end of the larger systems. Furthermore, telephone switching systems typically have a limited number of data ports and at least one such data port is required for each voice system that may be connected to a switch in a central office; the number of message systems connected to one central office may be several thousand. Furthermore, the arrangement using the special data link, as well as certain of the other prior art arrangements require special software in the switch which further adds to the expense of providing the service.